The present invention relates to an image processing method and apparatus for performing halftone conversion using a threshold mask so as to output an image to an output apparatus which has the number of gray levels smaller than that of an input image.
Conventionally, ordered dithering, a blue noise mask method, and the like are known as halftone processes for expressing a multi-valued (multi-level) image in a binary output apparatus. In these methods, the number of ON dots of an output image increases in proportion to the pixel value of an input image. For example, when an input image has 8-bit precision, the number of ON dots of an output image ranges from xe2x80x9c0xe2x80x9d to xe2x80x9c255xe2x80x9d pixels per 255 pixels with respect to an input value ranging from xe2x80x9c0xe2x80x9d to xe2x80x9c255xe2x80x9d. On the other hand, an output apparatus such as an ink-jet printer cannot obtain an output proportional to an input value due to mechanical and optical dot gains.
For this reason, a correction process for canceling the characteristics of the output apparatus is required. As correction methods, a method of halftoning an input value which is corrected using a look-up table (LUT) that outputs a corrected 8-bit value in response to an 8-bit input, a method using a threshold mask that has a nonlinear dot gain characteristic, and the like have been proposed.
However, in the conventional method using an LUT, when the numbers of levels of the input and output of the LUT are equal to each other, the number of gray levels of an input image may decrease as a result of looking up the LUT.
On the other hand, in the method using a threshold mask that has a nonlinear dot gain characteristic, threshold masks corresponding to, e.g., different output characteristics depending on the types of paper used in print processes are required.
Furthermore, when the characteristics of an input image are different from the target characteristics of correction, an additional correction process is required and, as a result, the number of gray levels of an input image cannot be maintained.
It is an object of the present invention to provide an image processing method and apparatus, which can solve the conventional problems, can perform halftone conversion corresponding to the tone reproduction characteristics of an output apparatus by a small memory size without decreasing the number of gray levels of an input image, and can output a high-quality image.
It is another object of the present invention to provide an image processing method and apparatus, which can correct the tone reproduction characteristics by a small memory size by generating a second threshold mask having threshold values corresponding to the number of gray levels, which is equal to that of an input image, from a first threshold mask having threshold values corresponding to the number of gray levels, which is larger than that of the input image, on the basis of tone reproduction characteristics corresponding to the type of paper used in a printer or the printer state.
It is still another object of the present invention to provide an image processing method and apparatus which can correct an image to have arbitrary input/output characteristics by a small memory size for a plurality of paper types having different output characteristics by generating threshold values of a daughter mask in accordance with an LUT having output characteristics corresponding to the type of paper used in a printer, and the relationship between the input value and output density.
It is still another object of the present invention to provide an image processing method and apparatus which can output a color image with an arbitrary color balance by a small memory size by generating threshold masks in units of colors in accordance with LUTs having output characteristics in units of colors corresponding to the type of paper used in a printer, and input/output characteristics designated in units of colors.
Other features and advantages of the present invention will be apparent from the following description taken in conjunction with the accompanying drawings, in which like reference characters designate the same or similar parts throughout the figures thereof.